This invention relates to a free space laser with monomode fibre output that exhibits therefore the advantage of taking into account, by its very construction, coupling with a monomode optic fibre of the luminous flux transmitted. The very structure of these lasers enables self-alignment of the luminous flux transmitted by the amplifying medium in free space with the output monomode fibre. Thus, the advantages of free space lasers are combined to those of guided lasers coupled to a leading fibre and those of fibre lasers.
Free space lasers 1 are well known and consist of a Fabry-Perot cavity comprising two mirrors 3, 4 and an amplifying medium 2. These free space lasers of the previous art are represented diagrammatically on FIG. 1.
In order to enable transmission of a luminous flux 8, one of the mirrors 4 is partially translucent, and also to ensure stable resonance of the cavity, one of the mirrors at least is concave. Indeed, these free space lasers are generally configured to transmit over the non-guided fundamental lateral mode TEM00 whose transversal intensity distribution is Gaussian in shape. This Gaussian mode of the laser can be coupled very efficiently in a monomode 6 whose guided mode has a shape very similar to a Gaussian. To do so, a lens 5 with focus distance f such that the waist of the converging beam exhibits the same length as the guided mode of the monomode fibre. The input face of the fibre is located on the focal plane of the lens and the core 7 of the fibre is centred on the waist of the convergent laser beam.
In order to enhance stability of the cavity, one of the mirrors 3 can be replaced with a self-aligned retroreflecting device such as a cube corner 9 or a cat""s eye 10, respectively represented on FIGS. 2 and 3. In such a case, the cavity is always optimised, but the direction of the output beam 8 is given by the orientation of the mirror 4 facing the self-aligned retroreflector.
By self-aligned retroreflecting, we mean here a device designed for receiving a collimated incident beam and for sending it back in the form of a reflected beam, also collimated, whereas the reflected beam exhibits, at least in the first order, a direction parallel to that of the incident beam, even if the beam shows a small angular deviation with respect to its theoretical incidence direction. This is in particular the case of the cube corners that are orthogonal trihedra whose inner faces are retroreflecting. The best operation is obtained for an incident beam, more or less parallel to the diagonal of the cube whose trihedron forms an apex. It is also the case of the cat""s eye consisting of a plane mirror arranged in the focal plane of a convergent lens. Best operation is obtained for an incident beam that is more or less parallel to the axis of the lens.
With these free space lasers, one of the mirrors can also be replaced with a wavelength dispersive retroreflecting system, comprising for instance a diffraction network 12 and a mirror 13; this is represented on FIG. 4. Such a device enables selection of a specific transmission wavelength, whereas the angular variation of the retroreflecting dispersive system enables providing a wavelength-tuneable laser.
For all these free space lasers, the output beam 8 can be coupled efficiently in a monomode fibre 6 if the laser also transmits in its fundamental mode TEM00. We know that the coupling conditions in the fibre are optimum when the axis 14 of the output laser beam 8 is aligned with the axis 15 delineated by the centre of the lens and the centre of the core 7 of the fibre and when the output face 16 of the fibre is in the focal plane 17 of the lens. This appears for instance on the enlarged representation of FIG. 5.
The stability of the coupling and henceforth of the power transmitted depends therefore, in particular, on the stability of the lens-core axis 15, but also on the axis 14 of the output laser beam. However, this beam suffers from instabilities due to residual instabilities caused when aligning the laser mirrors. Stabilisation is possible with counter-reaction mechanisms acting on the orientation of the mirrors but leading to a system of significant complexity.
The purpose of the invention is to suggest a free space laser with a monomode fibre output that is self-aligned to eliminate these instabilities.
To this effect, the invention therefore relates to a free space laser comprising an amplifying medium, in free space, located inside a Fabry-Perot cavity, whereas two reflectors make up this cavity, one of both reflectors being partially translucent and constituting the laser output mirror.
According to the invention, the output mirror is inserted in the core of a monomode optic fibre that constitutes an output optic fibre; a lens, located in the cavity, ensures coupling in this fibre of the luminous fluxes transmitted by the amplifying medium and the second reflector is self-aligned.
Advantageously, in various embodiments, each exhibiting its own advantages and liable to be combined:
the output mirror is a Brag network photo-inscribed in the core of the optic fibre;
the self-aligned reflector is a cube corner;
the self-aligned reflector is a cat""s eye;
the laser comprises means enabling to vary the transmission wavelength;
the laser comprises means ensuring variation continuity of the transmission wavelength;
the means ensuring variation of the transmission wavelength comprise a network operating in the Littman-Metcalf configuration;
the lens has a zero frontal distance;
the lens has an index gradient;
as the amplifying medium exhibits plane interfaces with the free space, the planes of these interfaces are tilted with respect to the normal line of the laser axis.